1. Field of the Invention
The present invention relates to a safety switch for a battery, and more particularly, to a charge/discharge regulator circuit for a lithium ion battery to prevent over-charge or over-discharge of the lithium ion cell.
2. Description of the Related Art
Electrochemical batteries are generally used to provide direct current and power in a large variety of different operations. Batteries utilizing the reactivity of lithium metal are well known. Batteries employing elemental lithium as the anode, however, may become hazardous under certain circumstances. Further research in this field led to the development of lithium ion batteries in which elemental lithium is replaced by substances intercalating lithium ions. Such intercalating substances are capable of absorbing substantial amounts of lithium ions and reversibly releasing the lithium ions in a subsequent operation. An example is trilithium nitride, whose structure consists of layers of dilithium nitride, between each of which is a layer of lithium atoms. This markedly increases the conductivity, so that the material becomes an effective solid electrolyte, for example, in a lithium ion battery.
A conventional lithium ion battery has a negative electrode comprising an active material which releases lithium ions when discharging and intercalates or absorbs lithium ions when the battery is being charged. The positive electrode of a lithium ion battery comprises an active material of a different nature, one that is capable of reacting with lithium ions on discharge, and releasing lithium ions upon charging the battery. It is well known that over-discharge of the lithium cell may result in dendritic or metal filaments growing from one side of the cell to the other, or across the intercalating membranes. This electrically conductive crystalline structure can short circuit the cell and permanently destroy the cell's operation. Over charging a lithium ion battery may cause not only destruction of the cell, but may also create a fire hazard. During recharge, if the recharging voltage exceeds a certain voltage called the over-charge limit, the recharging circuit may be putting too much energy into the battery. The battery will continue to charge, however, resulting the excess energy is converted to heat. Under these conditions, the lithium ion battery may become hot, it can vent gas, and potentially catch fire.
Accordingly, a lithium ion battery safety switch or charge regulator is needed that will prevent over-discharge of the lithium ion battery, thus preventing formation of electrically conductive dendrites, as well as to prevent over-charging of the lithium ion battery, thus preventing overheating of the battery and the resultant risk of combustion or explosion.
Various charge regulating circuits have been employed including the use of FETs as fuses should the over-charge limit be exceeded, or thermal switches that sense the temperature of the lithium ion battery and open the charging circuit in the event a threshold temperature is reached. These are "one-way" switches in the sense that they open the circuit in the event recharge limits are violated (e.g., excess voltage, or excess current). Other one way switches, such as a minimum voltage sensor, detect an over-discharge condition and open a switch to prevent further discharge of the lithium ion battery. Ideally, a charge regulator circuit is bi-directional; i.e., one that can conduct in two directions and can block current in two directions.
A bi-directional switch or voltage regulating circuit, typically integrated within the lithium ion battery casing, is shown in prior art FIG. 1. A bi-directional FET assembly 1 comprising two FET transistors M1 and M2, is connected in series with the cell 6 in the lithium ion battery pack. Typically, M1 and M2 are two discrete power MOSFETs. The MOSFETs shown are N channel devices connected in a common drain configuration, although they may be connected in a common source configuration as well. A control integrated circuit 2 controls the gates 11, 12 of the MOSFETs such that either M1 or M2 will open the circuit 3 across the terminals 4, 5 of the lithium ion battery cell 6. As shown in prior art FIG. 1, the IC control circuit includes precision voltage references 7 and 8, representing the operating range limits of the cell including minimum discharge voltage of the lithium ion cell and the maximum charging voltage of the cell. Voltage comparators compare the voltage across the terminals 4, 5 of the cell with the precision voltage references and will open the circuit in the event these limits are exceeded.
During normal operation, voltages across the battery terminals will lie between the minimum discharge voltage, typically 2.5 volts, and the over-charge voltage, typically 4.2 volts, as shown in FIG. 1. In this range the gates 11, 12 of both MOSFETs are positively biased allowing current to flow though the n-channel MOSFETs. However, should the voltage fall below 2.5 volts during discharge, the output of comparator 9 associated with the over-discharge protection (ODP) portion of the control circuit 2 will output a negative voltage, blocking the current in the ODP MOSFET M1, thus opening the circuit. The presence of clamping diode 13, intrinsic to most power MOSFET structures, is of no consequence since it is reversed biased. The fact that the clamping diode 14 in MOSFET M2 is forward biased is also of irrelevant since only one of the devices need be off in order to open the circuit. Similarly, should the charging voltage exceed the 4.2 volt reference of the over-charge protection (OCP) portion of the control circuit, the output of the OCP operational amplifier 10 will go negative, switching the current off through OCP MOSFET M2 and thus, opening the circuit. In the event of an over-charge condition, the most positive part of the circuit is the drain of the MOSFET thus reverse biasing the clamping diode 14 of the OCP MOSFET so that it does not conduct.
A disadvantage of the protection circuit of FIG. 1 is that the voltage references 7 and 8 require power which is drawn from the lithium ion battery. This power draw is constant, even during periods of non-use, thus creating a constant current drain on the battery resulting in either a reduced duty cycle or operating time, or the need to recharge the battery prior to use in the event of prolonged storage. Another disadvantage of the circuit of FIG. I is that lithium ion battery over-discharge and over-charge voltages are critical and must be maintained within a very narrow range, typically 80 millivolts. Further, these reference voltages are battery specific and must be intimately tied to the characteristics of the battery in which the circuit is employed. Exceeding the maximum charging voltage, even for a brief period of time may result in thermal runaway of the battery and its eventual destruction. These factors have resulted in development and commercialization of very complex control ICs comprising precision analog circuits, having very low current consumption over a wide temperature range. These ICs represent a significant portion of cost of the battery assembly and severely limit the commercial applications to which lithium batteries might be used.
An example of the lithium ion battery switch shown in Figure one is disclosed by Fernandez et al. in U.S. Pat. No. 5,539,299. They disclose a recharging circuit for a lithium ion battery that utilizes a control circuit for sensing over-charge conditions. Upon sensing an over-charge condition, the control circuit biases the gate of a power MOSFET to prevent current flow from the charging circuit to the battery. Typical of the device diagrammed in prior art FIG. 1, the device disclosed by Fernandez utilizes a control circuit comprising an operational amplifier and a voltage reference. These devices consume energy from the battery even when the load is disconnected.
Accordingly, there is a need for a protection switch for a lithium ion battery that will provide battery protection from over-discharge or over-charge condition comparable in performance to protection switches currently available except without the need for complex, active control circuits and have substantially no power drain on the battery.